1. Field of the Invention
This invention relates to a ceramic honeycomb filter for removing floating fine particles from exhaust gas.
2. Description of the Prior Art
Various conventional filters have been used for removing floating particles from gas; for instance, a filter with filler such as adsorbing material or the like arranged therein, steel wool, ceramic fiber, and other porous materials. In the case of filters to remove the fine particles, means for making the filtering meshes small are used in order to increase removing efficiency thereof, but such means result in a disadvantage that the pressure loss therein becomes large when a high filtration rate is required. Moreover, the structure of the conventional filters is limited to comparatively simple ones, e.g., sheet-formed, plate-formed, or cyclindrical filter, and filters of corrugated structure, double cylindrical form, or lamination type are used in order to make their filtering areas large. However, it has been difficult to remarkably increase the filtering areas of the conventional filters.
On the other hand, a honeycomb structural body has widely been used as a support of catalyst to clean automobile exhaust gas, a heat exchanger or the like. The honeycomb structural body has noticeable advantages in that fluid pressure loss therethrough is low because a multiplicity of parallel channels of certain configuration are uniformly disposed therein, that the surface area per unit volume thereof is large, and that partition walls between the adjacent channels can be made thin to allow easy heat-up thereof. The actual application of the honeycomb structure, however, has been limited to heat exchangers and catalyst supports.
It has been proposed to make a ceramic honeycomb filter by sealing selected channels of a ceramic honeycomb structure at one ends thereof while sealing the remainder of the channels at the opposite ends thereof in such a manner that dust-containing gas flows therethrough from said one ends to the opposite ends. Since every channel which is open at said one end is closed at the said opposite end, the gas has to pass through the partition walls. Namely, the partition walls which are porous serve as filtering means and filter the floating fine particles from the gas flowing therethrough.
Accordingly, sealant used in the proposed ceramic honeycomb filter should tightly adhere to the partition wall, so as to prevent the dust-containing gas from passing through the ceramic honeycomb filter without being filtered at the aforementioned partition walls. In case of using the ceramic honeycomb filter at a high temperature, both the sealant and the honeycomb structural body must have high heat resistances. More particularly, it is necessary to select proper sealing material from ceramic materials which hardly form liquid phase even at a temperature for firing the ceramic honeycomb structural body. With the ceramic honeycomb filters which have been proposed heretofore, when the ceramic honeycomb structural body with the sealing material applied thereto is fired and cooled, the partition walls tend to be cracked or the applied sealing material portions tend to shrink excessively to produce gaps between the partition walls and the sealing material, which gaps tend to cause the sealant to be separated from the honeycomb structural body. In short, the ceramic honeycomb filters of the prior art have shortcomings in that the difference of shrinkages between the partition walls and the sealing material during cooling tend to cause cracks of the partition walls and separation of the sealant from the ceramic honeycomb structural body. Due to such shortcomings, the use of the ceramic honeycomb filter has been limited heretofore.